Anechoic test chambers are used for conducting electromagnetic analyses. One use of an anechoic chamber includes testing loss or impedance properties of a material specimen. Loss properties that are often determined include impedance, permittivity, permeability, complex permittivity, and complex permeability. For example, S-parameter microwave analysis techniques are used to examine the changes in field strength, phase difference, and polarization effects of an electromagnetic wave propagated through a material specimen. From these analyses, the loss parameters are determined.
These techniques are used in many industries. The aerospace industry, for example, tests the loss characteristics of aircraft materials. In some cases, honeycomb structural composite materials are doped with electromagnetic attenuating material so that they have high loss characteristics. These characteristics may be used to define parameters for controlling radar cross sections.
The measurement of large sheet materials has been limited to microwave frequencies due to a lack of test apparatus that could properly illuminate the materials with a uniform electromagnetic wave in a confined space at lower frequencies. At lower frequencies, uniform electromagnetic fields are typically accomplished with waveguides. Thus, a material specimen is placed within the waveguide for the analysis to take place. As such, the material specimen must be cut for insertion. That is to say that these are destructive tests, the sample is cut in order to be precisely fitted into the waveguide and otherwise destroyed and unusable for manufacture after these lower frequency tests.
Anechoic test chambers are generally used for testing antennas that are terminated into a back wall. These anechoic chambers have not, however, been effective for low frequency material specimen testing due to the longer wavelengths at low frequencies. The longer wavelengths pose a problem of controlling a uniform field at the material specimen.
Therefore, what is needed is an anechoic test chamber that works down to sub-microwave frequencies. Such an anechoic test chamber may permit insertion of large samples without destruction. The test system should be capable of controlling a uniform electromagnetic field about the specimen.